1. Field of the Invention
The present invention relates to environmental cleanup of soil, underground water and atmospheric air which are polluted with the volatile chlorinated organic compound, and decomposition treatment of a volatile chlorinated organic compound generated in the industrial activities.
2. Related Art
A large amount of a volatile chlorinated organic compound (including chlorinated ethylene, chlorinated methane and the like, and hereafter abbreviated as,“VOC”), which was used at one time in industrial activities, has polluted soil and underground water, and require a great effort for purifying them. However, the VOC is still continuously used at present and continues polluting the air environment.
As for a process of effectively decomposing the VOC existing in such environments, a process of mixing a gas containing the VOC extracted from polluted soil, underground water and air, with a chlorine gas, and irradiating the gas mixture with light is proposed (Japanese Patent Application Laid-Open No. 2001-137697).
In addition, a process of decomposing the VOC adsorbed in an adsorbent by the same decomposing mechanism as in Japanese Patent Application Laid-Open No. 2001-137697 is proposed (Japanese Patent Application Laid-Open No. 2002-1062). The process includes the steps of: at first, adsorbing the VOC in a gas to be treated into an adsorbent such as activated carbon; subsequently, contacting the adsorbent with water vapor to desorb the VOC from the adsorbent; further condensing the water vapor; then, mix the condensate with a hypochlorous acid solution; and aerating a mixed solution, while irradiating it with light, to decomposes the VOC existing in the mixed solution.
In a process of decomposing a VOC through a decomposing mechanism of mixing chlorine with the VOC and irradiating it with light, according to Japanese Patent Application Laid-Open No. 2001-137697, the higher is a concentration of the VOC in a gas to be treated, the less is an amount of necessary chlorine per an amount of the VOC. The reason is considered to be that desorbed chlorine by the decomposition of the VOC also contributes to decomposition as well as charged chlorine, and that because the reaction uses a radical chain reaction, the higher is the concentration of chlorine existing in a reaction field, the more improved is a reaction efficiency. On the contrary, when the concentration of the VOC in a gas to be treated is low, an amount of necessary chlorine per an amount of the VOC largely increases to extremely decrease the treatment efficiency.
In general, in the case of soil and/or ground water pollution, pollutants which have penetrated into the underground from a polluted part diffuse to a wide area of the underground by an effect of the underground water, to pollute the wide area at a low concentration. For this reason, when the VOC is intended to be extracted from such a polluted site, a large amount of but a low concentration of the gas is extracted. When the gas is treated by optical function water, there has been a problem of increasing a treatment cost. The problem is to be solved also in air pollution and a VOC effluent gas produced in industrial activities.
A system according to Japanese Patent Application Laid-Open No. 2002-001062 includes the steps of temporarily collecting a VOC gas in an adsorbent; then desorbing the VOC from the adsorbent to obtain condensate containing the VOC with a high concentration; and subjecting the condensate to decomposition treatment. Accordingly, the above-described VOC-decomposition process can be applied to treatment for a large amount of a VOC gas having a low concentration.
However, though the above-described VOC-decomposing process needs to mix chlorine having a concentration corresponding teethe concentration of the VOC to be treated, with a gas to be treated, the process according to Japanese Patent Application Laid-Open No. 2002-1062 has problems that a concentration ratio of a VOC vaporized from condensate to chlorine is hardly controlled to a suitable ratio for a decomposition condition, because in the process, aeration is performed after a hypochlorous acid solution which is a chlorine evolution source has been mixed with condensate, and that the VOC gas to be treated cannot acquire so high concentration, because a large quantity of the aerating gas needs to be used for vaporizing the VOC from the condensate. The above process also has the problem of needing a long period of time for enabling an adsorbent which has desorbed a VOC to be used again (which is called “regeneration”), because the decomposition treatment cannot be started before the VOC completely desorbs from the adsorbent.
The present invention has been designed with respect to the above-described problems, and is directed at more efficiently treating a large amount of the VOC having a low concentration with a VOC-decomposing process of irradiating the VOC with light in the presence of chlorine.